Note
Make sure the device has at least 10% battery life
Do at your own risk!
Download an extract eMMC DL tool from here
Download an extract the Stock ROM of the device you wnat ot Flash or upgrade
Install Qualcomm USB drivers from the extracted folder
Connect the device to PC while holding volume up + volume down. (Some devices require holding only volume up or just volume down)
Once device is connected properly, go to device manager of your PC (Right Click on Computer > Properties > Device Manager)
Expand Port (COM & LPT) and note the port number of Qualcomm HS-Diagnostics
Now launch eMMC DL Tool
Input the COM Port number of Qualcomm HS-Diagnostics after Disk1
Click on FW Path browse button
Navigate and select folder containing the firmware of the device you want to flash
The firmware will be added sucessfully to eMMc DL Tool
Click DL1 to start flashing or upgrading process
You will see 2 green bars indicating flashing progress
When you see Green Pass, know that flash process is complete